The present invention concerns automotive electrical systems, and particularly circuits within that system for sensing and utilising an ignition voltage signal. More specifically, the invention concerns a circuit for compensating errors in the sensed ignition voltage signal.
Automotive control systems have become progressively more sophisticated. Most new vehicles rely upon many microprocessors or micro controllers for controlling various aspects of the vehicle function. One typical vehicle electrical system 10 as shown in FIG. 1. The system includes a power supply 11, which is typically the vehicle battery. A power bus 16 connects the battery to a number of electrical and electronic components. For example, the battery feeds power through the ignition switch 15, as well as to a power mode controller 17, and a radio 18. In addition, ancillary control modules are connected to the power bus 16, such as a power train control module 20, an airbag control module 21, an antilock braking system module 24, and additional customer supplied modules 22 and 23.
Each of these components performs various functions in the vehicle control system. For instance, the power mode module 17 is also sometimes referred to as a xe2x80x9cbody computerxe2x80x9d because it controls various active suspension and vehicle body functions. The power train module 20 provides control signals to components within the vehicle power train. The airbag control module 21, also known as the sensing and diagnostics module, controls the operation of forward and side airbags associated with the vehicle. The ABS module 24 includes a micro controller that provides control signals to the antilock or anti-skid braking system. Finally, the additional modules 22 and 23 can include microprocessors or micro controllers that perform other customer-selected vehicles and/or engine functions.
Although all of the modules within the electrical system 10 are supplied with power directly from the battery 11, the initiation of these modules can frequently depend upon the ignition state of the vehicle. Most vehicle ignition switches, such as the switch 15, have many operating positions. For example, the ignition switch 15 can be moved to an IGN1 position which is activated when the vehicle engine is in the run or crank mode. Alternatively, the ignition switch can be moved to an xe2x80x9caccessory positionxe2x80x9d in which a signal is provided on line 26. A third possible position for the ignition switch 15 is a xe2x80x9ccrankxe2x80x9d position in which the vehicle engine is being cranked prior to actually starting. In this condition, the ignition switch provides a signal on line 27 that can be used by the power train control module 20 to perform various engine-cranking functions.
In addition to starting the engine, placing the ignition switch 15 in the IGN1 position also generates a voltage signal on signal line 25 that is used by other electronic modules. Specifically, some of the modules are only activated when the vehicle engine is started and running. When the engine has stopped, these modules can be required to move to a different operating mode.
Thus, as shown at FIG. 1, the voltage signal IGN1 on line 25 is provided to the powertrain control module 20 on line 25A, the airbag control module 21 on line 25B, the customer supplied module 23 on line 25C, the ABS module 24 on line 25D, and to the power mode module 17 on line 25E. Each of these modules relies on an accurate voltage for the signal IGN1 to determine the mode of operation for the particular module. In one specific example, the airbag module control 21 has an active and inactive state. In the active state, the module 21 provides control signals to the airbag components to permit their operation in the event of a vehicle crash. In its inactive state, the module 21 essentially deactivates the airbag system. To insure the safety of the occupants, the airbag control module 21 is in its inactive condition at least until the vehicle engine is running. In order to make this determination; the module 21 reads the signal IGN1 on signal line 25B. If that signal exceeds a predetermined threshold voltage, it is assumed that the ignition switch 15 is in its xe2x80x9crun/crankxe2x80x9d position and that the engine is in fact running.
However, as vehicle electrical systems become more complex, the actual voltage of the ignition signal IGN1 may be subject to transient fluctuations. It is therefore been necessary to incorporate active circuit components that receive and evaluate the ignition signal IGN1 to determine the on/off state of the vehicle ignition. In one typical system, a forward biased diode and resistor circuit is utilized to prevent negative transients from affecting the output voltage value. While this resistor-diode network addresses the problem of negative transients, it also introduces a certain degree of non-linearity and unpredictability. Some microcontrollers or electronic modules can handle widely varying ignition voltage signals. However, many other modules are more sensitive and require a more tightly toleranced voltage signal to be evaluated.
There is therefore a need for an ignition sensing system that addresses external transients that impact voltage signal without adding new errors to the output voltage signal.
In response to this need, the present invention provides a compensation circuit for use with an ignition voltage sensing circuit. The ignition sensing circuit includes an active filter element in series with a resistance element, which is configured to filter or block transients superimposed on the ignition voltage signal. In accordance with the preferred embodiment of the invention, the sensing circuit includes a forward biased diode and a resistor connected between an input receiving the ignition voltage signal and an output node. A second resistor is connected between the output node and ground. Prior to introduction of the inventive compensation circuit, the voltage signal at the output node is provided to a microprocessor of a control device that executes power molding based on the magnitude of the ignition voltage signal.
In accordance with one aspect of the invention, a compensation circuit includes a second active element, such as a diode, in series between the second resistor and ground. In an important feature of the invention, the second active element has substantially identical electrical properties and performance characteristics as the active filter element. In a specific embodiment, both elements constitute substantially identical diodes mounted on a common substrate. Thus, the voltage drop across both diodes is expected to be substantially identical under all environmental conditions, such as temperature.
The present invention capitalizes on the identity in diode performance to compensation for voltage errors in the sensed ignition voltage signal introduced by the active filter element. Thus, in accordance with a further feature of the invention, means are provided for subtracting the voltage drop across the compensation diode from the voltage signal at the first output node of the filter circuit. In the preferred embodiment, this means constitutes software instructions implemented by the microprocessor of the device acting on the ignition voltage signal. These software instructions implement the following equation based on particular values for the two resistors in the filter circuit: IGN1=4xc3x97(IGN_D1xe2x88x92(IGN_D2)+2), where IGN1 is the corrected ignition voltage, IGN_D1 is the voltage at the first output node, and IGN_D2 is the voltage at a node between the compensation diode and the second resistor. The corrected ignition voltage value can then be provided to the power moding and testing components of the device microprocessor.
It is one object of the invention to provide an ignition voltage sensing device that can eliminate unnecessary transient signals from the actual ignition voltage. A further object is achieved by features of the invention that compensates for or overcomes errors introduced into the sensed voltage signal by the voltage sensing device.
One benefit of the invention is that it is easily implemented within existing ignition voltage sensing devices. A further benefit is accomplished by aspects of the invention that addresses environmental effects on the voltage sensing device to provide an accurate signal to other devices relying upon ignition voltage.
These and other objects and benefits will become apparent upon consideration of the following written description of the present invention, together with the accompanying figures.